Microwave ovens have used cavities containing mode stirring structures to provide varying electric field patterns in the cooking area of the oven by introducing microwave energy into a cavity, which generally has interior dimensions large with respect to a wavelength of the microwave frequency, and moving conductive elements in the cavity to reflect the energy and vary the patterns so that points of maximum voltage gradients are continuously shifted in the cavity to more uniformly heat different sizes and shapes of bodies. Such a reflective mode stirrer, which is designed for one set of load conditions such as heating hamburgers or hot dogs, does not produce the same effectiveness in uniformly heating a large body such as a roast or a wide relatively planar body such as a pie.
Coaxial feeds of microwave energy into microwave heating cavities with mode stirrers rotating concentric with the feed have still provided that the mode stirring be by reflection from metal members moving with respect to the food body as shown, for example, in U.S. Pat. No. 3,436,507, issued on Apr. 1, 1969 to H. A. Puschner.